Dear all,
I am currently in the fourth year of my applied physics studies and I am asked to do research on the Polywell topic. I was very happy when I found this forum. Up to now I found a lot of useful information for my thesis. Currently I set some new research questions regarding the fuel for the reactor. I would very much like some help and insight on answering these questions (i.e. some research papers or a brief opinion).
- I am asked to study the supply chain and production process for boron used during Polywell fusion (mines, seawater etc.)
- The focus lies on the specifications needed for the boron in order to be used as fuel for the reactor
- The result should give an overview of the available sources and the global supplies
Thanks in advance
Research questions on Boron fuel for Polywell reactor
Re: Research questions on Boron fuel for Polywell reactor
There are three issues with handling Boron as I see it.
The desired Boron isotope is Boron11, while the most common isotope is Boron10. Isotope separation is needed, and it is not a a new issue. Boron 11 isotopes have been seperated and purified for several reasons. Boron 10 is an excellent neutron absorber and has applications in certain types of neutron counters, and in radiation resistant electronics (if I remember right).
Boron is not a gas at STP type conditions. The probable solution is to use decoboranes if boron ions are sourced from a gas feed, as opposed to erosion of some solid source.
Boron, not being a gas at low energy levels- like near room temperature of even boiling water temperatures, may lead to vacuum chamber depositions that may have consequences for isolaters, etc. This issue is not limited to boron. Hydrogen also can be embedded into surfaces and have serous consequences for materials durability (think hydrogen belittlement). Liquid lithium handling inside a vacuum vessel will be difficult.
Once the physicists hopefully achieve effective and profitable fusion, the engineers will have to solve a multitude of challenges to make a system actually work as a commercial product. I think the tokamak approach is getting very close to physics success (on their slow timescale), but the engineering challenges are formidable, and many think unobtainable for a viable commercial and economical application. Based on my admitted weak understanding, I suspect the Polywell and General Fusion approaches may be the most feasible from an engineering viewpoint. General Fusion because it is a brute force issue with the liquid lead serving multiple purposes. And, the Polywell because it's magnetic properties also applies directly to some of the first wall issues- heat and erosion. I believe a Field Reversed cobfiguration will have a diverter problem similar to tokamaks, or spheromaks (?).
The physics issue with boron is mostly centered around Bremsstruhlung radiation due to Boron having a Z of 5 and Bremsstruhlung radiation scaling as the square of Z. Rider felt this was a show stopper, but there are proposed workarounds, mostly based on three aspects. First the dynamics of the electron energy changes at different radii in the machine.Bremsstruhlung is due to fast electrons interacting with ions. If there is confluence towards the center of the ions due to potential well and the spherical geometry, the fast electron will interact with relatively less dense ion populations. Where the ion density is greatest in the center, the electrons are slow and thus produce less Bremsstruhlung, especially relative to the amount of fusion occuring in this central core. Thermalization or (partial) lack thereof also changes the picture . Finally, diluting the boron to perhaps ~ 1 boron for each 10 hydrogens will change the average Z of the mixture.
Dan Tibbets
The desired Boron isotope is Boron11, while the most common isotope is Boron10. Isotope separation is needed, and it is not a a new issue. Boron 11 isotopes have been seperated and purified for several reasons. Boron 10 is an excellent neutron absorber and has applications in certain types of neutron counters, and in radiation resistant electronics (if I remember right).
Boron is not a gas at STP type conditions. The probable solution is to use decoboranes if boron ions are sourced from a gas feed, as opposed to erosion of some solid source.
Boron, not being a gas at low energy levels- like near room temperature of even boiling water temperatures, may lead to vacuum chamber depositions that may have consequences for isolaters, etc. This issue is not limited to boron. Hydrogen also can be embedded into surfaces and have serous consequences for materials durability (think hydrogen belittlement). Liquid lithium handling inside a vacuum vessel will be difficult.
Once the physicists hopefully achieve effective and profitable fusion, the engineers will have to solve a multitude of challenges to make a system actually work as a commercial product. I think the tokamak approach is getting very close to physics success (on their slow timescale), but the engineering challenges are formidable, and many think unobtainable for a viable commercial and economical application. Based on my admitted weak understanding, I suspect the Polywell and General Fusion approaches may be the most feasible from an engineering viewpoint. General Fusion because it is a brute force issue with the liquid lead serving multiple purposes. And, the Polywell because it's magnetic properties also applies directly to some of the first wall issues- heat and erosion. I believe a Field Reversed cobfiguration will have a diverter problem similar to tokamaks, or spheromaks (?).
The physics issue with boron is mostly centered around Bremsstruhlung radiation due to Boron having a Z of 5 and Bremsstruhlung radiation scaling as the square of Z. Rider felt this was a show stopper, but there are proposed workarounds, mostly based on three aspects. First the dynamics of the electron energy changes at different radii in the machine.Bremsstruhlung is due to fast electrons interacting with ions. If there is confluence towards the center of the ions due to potential well and the spherical geometry, the fast electron will interact with relatively less dense ion populations. Where the ion density is greatest in the center, the electrons are slow and thus produce less Bremsstruhlung, especially relative to the amount of fusion occuring in this central core. Thermalization or (partial) lack thereof also changes the picture . Finally, diluting the boron to perhaps ~ 1 boron for each 10 hydrogens will change the average Z of the mixture.
Dan Tibbets
To error is human... and I'm very human.
Re: Research questions on Boron fuel for Polywell reactor
This link gives some paper sources from Rider and Chacon.
viewtopic.php?f=3&t=5354
Askmar.com provides many of Bussard's papers and associated papers.
Dan Tibbets
viewtopic.php?f=3&t=5354
Askmar.com provides many of Bussard's papers and associated papers.
Dan Tibbets
To error is human... and I'm very human.
Re: Research questions on Boron fuel for Polywell reactor
> I believe a Field Reversed cobfiguration will have a diverter problem similar to tokamaks, or spheromaks (?).
What kind of divertor problem are you thinking of? One of the major advantage of the FRC is the scrape-off layer which naturally occurs from the formation of the FRC. In fact, I would say that it contains some of the "ideal" divertor characteristics, including the detachment of the core from the SOL, and the fact that it makes contact with a material target as far as you would want so that impurities won't affect your hot core in the same way as it would in a tokamak divertor.
>Based on my admitted weak understanding, I suspect the Polywell and General Fusion approaches may be the most feasible from an engineering viewpoint.
I agree about General Fusion because there has been enough experimental work on similar conditions in several different institutes/labs, including GF. However, you can't put Polywell on the same level, not if you're putting FRC and Tokamaks as "less feasible". Polywell is yet unproven, and there isn't anywhere near the level of work done on it to make that assumption. (The level of work doesn't mean we should do less, but it needs to be acknowledged.)
What kind of divertor problem are you thinking of? One of the major advantage of the FRC is the scrape-off layer which naturally occurs from the formation of the FRC. In fact, I would say that it contains some of the "ideal" divertor characteristics, including the detachment of the core from the SOL, and the fact that it makes contact with a material target as far as you would want so that impurities won't affect your hot core in the same way as it would in a tokamak divertor.
>Based on my admitted weak understanding, I suspect the Polywell and General Fusion approaches may be the most feasible from an engineering viewpoint.
I agree about General Fusion because there has been enough experimental work on similar conditions in several different institutes/labs, including GF. However, you can't put Polywell on the same level, not if you're putting FRC and Tokamaks as "less feasible". Polywell is yet unproven, and there isn't anywhere near the level of work done on it to make that assumption. (The level of work doesn't mean we should do less, but it needs to be acknowledged.)
Re: Research questions on Boron fuel for Polywell reactor
From thermal load aspects, I understand the FRC scrape off layer is equivalent to the tokamak diverter. Am I wrong?
My optimism for the Polywell is not only from a proven experimental physics aspect. It is from my expectations due to the favored relaxed conditions inherent in the design. MHD stability and magnetic shielding, thus lower thermal loads for the most potentially exposed elements- the magrid. Reasonable possibility for more efficient energy extraction- direct conversion. Reasoned physics expectations counter some of the criticisms. The dynamic energy distribution of ion and electron energies based on radius changes the bremsstruhlung picture, ion edge annealing may significantly delay ion thermalization. What worries me the most about the Polywell is the question of electron injection efficiency.
The experimental demonstration of the Wiffleball effect- high Beta operation with impressive to outstanding increases in cusp confinement is demonstration of the base essential physics consideration. Over the years there has been opposition to Bussard's claims on this forum about the Wiffleball effect- the effects on cusp confinement and its mechanism. That Dr Parks revealed the early work by Grad, etel. dilutes that argument. Also, the attainment of deep potential wells has been demonstrated for ~ 20 years, at least to EMC2/ Dr Park's satisfaction. This combined with the demoed Wiffleball effect leaves the question of energy costs to establish these conditions- mostly electron injection efficiency as the greatest physics question.
Bremsstrulung mitigation issues are critical for advance fuel viability-like P-B11, but for D-D fuel it is not a potential show stopper, even lack of convergence is not a show stopper according to Dr Nebel. Even Rider admitted that profitable D-D fusion in a Polywell type device was not an impossibility (as opposed for his claims for P-B11 fusion- based on his challenged assumptions). And, D-D fusion is much better than being limited to D-T fuel and all of it's complications. Of course, if D-D fusion is on the table, the implication is that D-T fusion is easy (ignoring the tritium generation challenge).
I think that earlier work on Beta=1conditions by Bussard, etel was flawed. He assumed that the peak plasma brightness was obtained as Beta=1 was traversed. His brightness measurements by photometry may have been OK- Dr Nebel said they matched another measurement modality. The problem may be that Bussard assumed constant inputs, except for B field strength. This would result in a peak brightness at Beta=1. But, it has become obvious that electron injection efficiency is not a constant over a range of B field strengths. Work by EMC2 and the Australian Sidney group has implied this. If electron injection efficiency drops off as the B field increases- the cusps loss/ entry cone shrinks, then the brightness peak will shift, perhaps a small amount to a large amount. It is uncertain how close Dr Bussard was coming to Beta=1. This is why Dr Park's work is good. It measures plasma "brightness" or rather density by an independent effect of actually contained high energy electrons and the expected effects on bremsstruhlung output. There is no question of input and output balance, it is a direct measurement of contained electron effects.
Note that any fusion scheme benefits from more efficient thermal to electrical conversion methods. The supercritical CO2 systems may be ~ twice as efficient as the water steam modality. Then there is Eric Learner's scheme for direct converting x-rays to useful electricity, perhaps at high efficiency. This also mitigates high bremsstruhlung loss concerns from a physics perspective (Q balance of the system). The engineering aspects may be a different matter...
Dan Tibbets
My optimism for the Polywell is not only from a proven experimental physics aspect. It is from my expectations due to the favored relaxed conditions inherent in the design. MHD stability and magnetic shielding, thus lower thermal loads for the most potentially exposed elements- the magrid. Reasonable possibility for more efficient energy extraction- direct conversion. Reasoned physics expectations counter some of the criticisms. The dynamic energy distribution of ion and electron energies based on radius changes the bremsstruhlung picture, ion edge annealing may significantly delay ion thermalization. What worries me the most about the Polywell is the question of electron injection efficiency.
The experimental demonstration of the Wiffleball effect- high Beta operation with impressive to outstanding increases in cusp confinement is demonstration of the base essential physics consideration. Over the years there has been opposition to Bussard's claims on this forum about the Wiffleball effect- the effects on cusp confinement and its mechanism. That Dr Parks revealed the early work by Grad, etel. dilutes that argument. Also, the attainment of deep potential wells has been demonstrated for ~ 20 years, at least to EMC2/ Dr Park's satisfaction. This combined with the demoed Wiffleball effect leaves the question of energy costs to establish these conditions- mostly electron injection efficiency as the greatest physics question.
Bremsstrulung mitigation issues are critical for advance fuel viability-like P-B11, but for D-D fuel it is not a potential show stopper, even lack of convergence is not a show stopper according to Dr Nebel. Even Rider admitted that profitable D-D fusion in a Polywell type device was not an impossibility (as opposed for his claims for P-B11 fusion- based on his challenged assumptions). And, D-D fusion is much better than being limited to D-T fuel and all of it's complications. Of course, if D-D fusion is on the table, the implication is that D-T fusion is easy (ignoring the tritium generation challenge).
I think that earlier work on Beta=1conditions by Bussard, etel was flawed. He assumed that the peak plasma brightness was obtained as Beta=1 was traversed. His brightness measurements by photometry may have been OK- Dr Nebel said they matched another measurement modality. The problem may be that Bussard assumed constant inputs, except for B field strength. This would result in a peak brightness at Beta=1. But, it has become obvious that electron injection efficiency is not a constant over a range of B field strengths. Work by EMC2 and the Australian Sidney group has implied this. If electron injection efficiency drops off as the B field increases- the cusps loss/ entry cone shrinks, then the brightness peak will shift, perhaps a small amount to a large amount. It is uncertain how close Dr Bussard was coming to Beta=1. This is why Dr Park's work is good. It measures plasma "brightness" or rather density by an independent effect of actually contained high energy electrons and the expected effects on bremsstruhlung output. There is no question of input and output balance, it is a direct measurement of contained electron effects.
Note that any fusion scheme benefits from more efficient thermal to electrical conversion methods. The supercritical CO2 systems may be ~ twice as efficient as the water steam modality. Then there is Eric Learner's scheme for direct converting x-rays to useful electricity, perhaps at high efficiency. This also mitigates high bremsstruhlung loss concerns from a physics perspective (Q balance of the system). The engineering aspects may be a different matter...
Dan Tibbets
To error is human... and I'm very human.
Re: Research questions on Boron fuel for Polywell reactor
>From thermal load aspects, I understand the FRC scrape off layer is equivalent to the tokamak diverter. Am I wrong?
I think it's important to keep in mind that you can (somewhat indefinitely) spread the area that of the SOL being splayed out in the FRC because the SOL ends where your device ends. You can't do that with tokamaks.
>The experimental demonstration of the Wiffleball effect- high Beta operation with impressive to outstanding increases in cusp confinement is demonstration of the base essential physics consideration.
The experimental measurements were suggestive, but not conclusive, at least, to my advisor. I missed that presentation because I was out of town, but I think it's good reason to push forward and do more work but not as definitive as a "demonstration"..
Again, I don't think that the polywell is an unappealing device; I just think that it's pre-mature to make such grandstanding assumptions about its viability, similar to the pre-mature announcements and glorification of Lockheed Martin's yet-unpublished-non-quantitative preaching. If we were really concerned about making progress in fusion research, we need to be more careful about how to proceed. Making overly optimistic statements then having to retract your words is how the sentiment of "always 20 years away" has come to settle in the minds of those who fund us. That is how we have ended up with promising fusion projects losing funding. It has little to do with tokamaks' successes and much more to do with these promises that are continually delayed.
I think it's important to keep in mind that you can (somewhat indefinitely) spread the area that of the SOL being splayed out in the FRC because the SOL ends where your device ends. You can't do that with tokamaks.
>The experimental demonstration of the Wiffleball effect- high Beta operation with impressive to outstanding increases in cusp confinement is demonstration of the base essential physics consideration.
The experimental measurements were suggestive, but not conclusive, at least, to my advisor. I missed that presentation because I was out of town, but I think it's good reason to push forward and do more work but not as definitive as a "demonstration"..
Again, I don't think that the polywell is an unappealing device; I just think that it's pre-mature to make such grandstanding assumptions about its viability, similar to the pre-mature announcements and glorification of Lockheed Martin's yet-unpublished-non-quantitative preaching. If we were really concerned about making progress in fusion research, we need to be more careful about how to proceed. Making overly optimistic statements then having to retract your words is how the sentiment of "always 20 years away" has come to settle in the minds of those who fund us. That is how we have ended up with promising fusion projects losing funding. It has little to do with tokamaks' successes and much more to do with these promises that are continually delayed.
Re: Research questions on Boron fuel for Polywell reactor
asdfuogh wrote:....
The experimental measurements were suggestive, but not conclusive, at least, to my advisor. I missed that presentation because I was out of town, but I think it's good reason to push forward and do more work but not as definitive as a "demonstration".. .
The experiment by Dr Parks was admittedly a coarse approach with minimal funds and time constraints. This is partially why the results are remarkable. Admittedly, the results ideally should be codified and peer reviewed in a journal article, but short of that one must analyze the results as presented. The real question is what could have given the measured signal if not bremsstruhlung from contained hot electrons and that the increased signal was due to increased density. Increased density being the product of hot electron input rate/ hot electron loss rate. The electron input rate was at worst constant-might have fallen some for several reasons, but I know of no way it would have increased*. The loss rate is a combination of cooling and electron escape primarily through cusps. ExB losses could have contributed some (probably no more than 10% or less).
An arguement could be made that the X-ray signal was due to constant hot electron populations, but increased ion targets for the bremsstruhlung interaction due to plasma injection. The argument against this is the time delay. It is apparently consistent with hot electron density buildup due to hypothesized improved hot electron containment. The injected plasma was cold (reportedly ~ 10 eV). Dr Parks has stated that there was little electron thermalization between the two electron populations due to the temperature differential. I am not certain how this is calculated, but it seems likely that if there was significant thermalization between the electron populations in the timeframe of the expirement would have resulted in the hot electrons being cooled to much colder temperatures , well below 2000 eV, because the density of the cold electrons that was part of the cold plasma injection would extremely exceed that of the injected hot electrons. The cutoff filter that blocked any x-rays (from bremsstruhlung or any other source like cold electrons hitting walls) lower 2000 eV prevents any of the cool electrons from participating in measured x-ray results. The directional sensing also eliminated x-ray sources outside of the core of the machine- ie electron wall impact generated x-rays were mostly eliminated as a possibility (except a small spot on the opposite wall that would have been within view of the sensor). I suppose an argument could be made that this small spot of hot electron escaped (or rejected) impacts could produce the energetic x-rays above 2000 eV, but I suspect the intensity possible and the timing results would not fit this explanation.
Then there was the demonstrated magnetic field strength reduction. I guess this is consistent with the expected exclusion of the electromagnet B field as Beta= 1 is approached. I am unaware of any alternate explanations.
The experiment seems to eliminate alternate explanations for the results, ergo...
* I don't know of any explanation for the results with the stated conclusions, except for fraud, or instrumentation/ design flaws. None of this is apparent to me. The confounding system contamination with high Z ions like tungsten might cloud the results, but Dr Parks seems to have addressed this with timing considerations and spectroscopic measurements. Fraud possibilities can only be fully eliminated by independent verification. This fits in with my often preached advantage of small machines like this. If anyone was truly interested, the experiment could be repeated, possibly with refinements, within a few months and with a minimal investment. Much of the equipment may already be sitting in a University lab. It took Dr Parks team a year to design, and build, and refine the experiment, but with the advantage of hindsight the time investment could be significantly shortened.
Dan Tibbets
To error is human... and I'm very human.
Re: Research questions on Boron fuel for Polywell reactor
I am honestly quite new to this so any help is welcome.
I have some follow up questions.
- Boron is not a gas at STP type conditions. The probable solution is to use decoboranes if boron ions are sourced from a gas feed, as opposed to erosion of some solid source.
You mean decaborane right? Would this have any consequences on the reaction?
Erosion can indeed be a problem. Do you know of any references made by for instance Bussard on this?
- And, the Polywell because it's magnetic properties also applies directly to some of the first wall issues- heat and erosion.
Could you maybe specify this? What do you mean with 'the first wall issues- heat and erosion'?
Do you maybe have a reference to the workarounds for bremsstralung?
I have some follow up questions.
- Boron is not a gas at STP type conditions. The probable solution is to use decoboranes if boron ions are sourced from a gas feed, as opposed to erosion of some solid source.
You mean decaborane right? Would this have any consequences on the reaction?
Erosion can indeed be a problem. Do you know of any references made by for instance Bussard on this?
- And, the Polywell because it's magnetic properties also applies directly to some of the first wall issues- heat and erosion.
Could you maybe specify this? What do you mean with 'the first wall issues- heat and erosion'?
Do you maybe have a reference to the workarounds for bremsstralung?
Re: Research questions on Boron fuel for Polywell reactor
This thread has some major head scratching, and some links.
viewtopic.php?t=1310
I should point out that I have sometimes mentioned Nolan as a counter to Riders conclusions, and this is not the case. Actually, it is Dolan that is the major paper that counters Rider.
viewtopic.php?f=3&t=5354
This site may also, be useful
http://thepolywellblog.blogspot.com/201 ... ument.html
Decaborane, or whatever spelling is correct, is a family of compounds of boron and hydrogen, As such, once atomized and ionized it would provide a convenient mixture of boron (purified B11 isotope) and protons (hydrogen nucleii), with an excess of protons which is probably desirable for bremsstruhlung reasons. Topping off the mixture with hydrogen gas to make the boron to proton ratio of1:10 is straight forward.
The erosion of some electrode material and vapor deposition onto another surface is a whole complex field of coating technologies. The prototype for me is vapor coating telescope mirrors with aluminum. The effects can be desirable or undesired. The magnitude of the problems have not been often discussed. It is one of those inconvenient engineering questions that at are swept under the rug until the physics issues are solved. Bussard did little to address then in his experiments. They are generally not a major concern for short pulsed experiments. Bussard was certainly cognizant of the issues, but they were of little concern to the questions of the physics. Bussard was involved ith the Riggatron and the wall damage issues were not ignored.
Dan Tibbets
viewtopic.php?t=1310
I should point out that I have sometimes mentioned Nolan as a counter to Riders conclusions, and this is not the case. Actually, it is Dolan that is the major paper that counters Rider.
viewtopic.php?f=3&t=5354
This site may also, be useful
http://thepolywellblog.blogspot.com/201 ... ument.html
Decaborane, or whatever spelling is correct, is a family of compounds of boron and hydrogen, As such, once atomized and ionized it would provide a convenient mixture of boron (purified B11 isotope) and protons (hydrogen nucleii), with an excess of protons which is probably desirable for bremsstruhlung reasons. Topping off the mixture with hydrogen gas to make the boron to proton ratio of1:10 is straight forward.
The erosion of some electrode material and vapor deposition onto another surface is a whole complex field of coating technologies. The prototype for me is vapor coating telescope mirrors with aluminum. The effects can be desirable or undesired. The magnitude of the problems have not been often discussed. It is one of those inconvenient engineering questions that at are swept under the rug until the physics issues are solved. Bussard did little to address then in his experiments. They are generally not a major concern for short pulsed experiments. Bussard was certainly cognizant of the issues, but they were of little concern to the questions of the physics. Bussard was involved ith the Riggatron and the wall damage issues were not ignored.
Dan Tibbets
To error is human... and I'm very human.
Re: Research questions on Boron fuel for Polywell reactor
First wall issues are mostly heat load issues. In a tokamak, the first wall is a liquid blanket of lithium, perhaps with some lead and/ or beryllium mixed in. This would protect the vacuum vessel walls. But, the macro instabilities and other high speed particle0 confinement escapees blasts the lithium, and sputtering seems likely. How is this contamination of the plasma handled?
Since the tokamak is a large machine there is a lot of heat impingement upon the first wall but because of the large surface area the thermal loads are presumably manageable. My working expectation (from M. Simon) is that 1-2 MW of thermal load heat handling is well within current engineering capabilities. 30-40 MW thermal loads such as that at tokamak diverters is not.
In the Polywell , the small size would seem to imply increased thermal wall loads. There are mitigating considerations. The magrid, which is the closet component to the confined plasma, is exposed to x-rays, ExB diffusion ions (minimal) and electrons, and neutrons if D-T or D-D is used. The magrid though, is protected from most of the impacts from very energetic fusion charged particles like He3, Tritium, alphas,as they can only escape confinement through cusps and thus deposit their energy on the more distant vacuum vessel walls (or are direct converted). Thus the thermal wall loading issues are more complex and have to be figured for various elements/ radii in the machine. The thermal wall loading on the magrid is lessened to a modest to major extent. I, being a Polywell fanboy, think that the physics may allow for substantial fusion power in small packages, and that the engineering issues of thermal wall loads may limit the size of the machines beyond the physics of B^4 * r^3 fusion power scaling. This is another area where PB11 would have a substantial advantage. If direct conversion can handle most of the energy of the produced alphas then the thermal wall loading issues is mitigated and X-ray thermal loads may be the single greatest size constraint.
ie: Conditions to produce the desired fusion power over input power may exceed the engineering tolorances of the materials and systems. Thus the system would require detuning to reach a compromise. Much like you might derate a jet engine for cost, and durability concerns.
Dan Tibbets.
Since the tokamak is a large machine there is a lot of heat impingement upon the first wall but because of the large surface area the thermal loads are presumably manageable. My working expectation (from M. Simon) is that 1-2 MW of thermal load heat handling is well within current engineering capabilities. 30-40 MW thermal loads such as that at tokamak diverters is not.
In the Polywell , the small size would seem to imply increased thermal wall loads. There are mitigating considerations. The magrid, which is the closet component to the confined plasma, is exposed to x-rays, ExB diffusion ions (minimal) and electrons, and neutrons if D-T or D-D is used. The magrid though, is protected from most of the impacts from very energetic fusion charged particles like He3, Tritium, alphas,as they can only escape confinement through cusps and thus deposit their energy on the more distant vacuum vessel walls (or are direct converted). Thus the thermal wall loading issues are more complex and have to be figured for various elements/ radii in the machine. The thermal wall loading on the magrid is lessened to a modest to major extent. I, being a Polywell fanboy, think that the physics may allow for substantial fusion power in small packages, and that the engineering issues of thermal wall loads may limit the size of the machines beyond the physics of B^4 * r^3 fusion power scaling. This is another area where PB11 would have a substantial advantage. If direct conversion can handle most of the energy of the produced alphas then the thermal wall loading issues is mitigated and X-ray thermal loads may be the single greatest size constraint.
ie: Conditions to produce the desired fusion power over input power may exceed the engineering tolorances of the materials and systems. Thus the system would require detuning to reach a compromise. Much like you might derate a jet engine for cost, and durability concerns.
Dan Tibbets.
To error is human... and I'm very human.